High-frequency furnace for high-frequency heating by means of ultra-high frequencies



1967 F. J. H.T|MMERMANS ETAL 3,304,399

HIGH-FREQUENCY FURNACE FOR HIGH-FREQUENCY HEATING BY MEANS OF ULTRA-HIGH FREQUENCIES Filed March 10, 1964 INVENTORS FRA/VC/SCUS JOSEPHUS HE/VDH/CUS T/MMERMANS JOl-IAN P05 THUMA VAN DER HE LM United States Patent C 3,304,399 HlGH-FREQUENCY FURNACE FQR HIGH- FREQUENCY HEATING BY MEANS OF ULTRA-HIGH FREQUENQEES Franeiscus Josephus Hendricus Timmermans, Hamburg, Germany, and Johan Posthuma van der Helm, Geldrop, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Mar. 10, 1964, Ser. No. 350,698 Claims priority, application Netherlands, Mar. 11, 1963, 290,076 8 Claims. (Cl. 21910.55)

The invention relates to dielectric heating devices which utilize ultra-high frequency energy. More particularly, to a high-frequency furnace provided with several ultra-high frequency generators wherein the ultrahigh frequency oscillations of each of the ultra-high frequency generators are applied to one of several folded waveguide systems arranged in sequence. Each waveguide system comprises a plurality of adjacent lengths of waveguide, while a channel is provided in all the waveguide systems through which the object to be heated is passed through the various waveguide systems in sequence in a direction at right angles to the walls of the adjacent lengths of waveguide, which for this purpose are each provided with an aperture of passage. The high-frequency furnace according to the invention may be used to particular advantage for drying foils, tows of filaments, and the like.

It is an object of the present invention to provide a particularly advantageous device of the kind described in the preamble, in which the heating efficiency is increased with a decrease in the size of the high-frequency furnace and furthermore the uniformity of the drying process is materially improved.

The high-frequency furnace according to the invention is characterized in that ultra-high frequency oscillations from each of the various ultra-high frequency generators are applied to the associated waveguide system through an additional folded waveguide system, which also comprises a plurality of adjacent lengths of waveguide provided with apertures in their walls for guiding the object to be heated in a direction at right angles to these walls. In order to effect high-frequency heating of the objects to be heated after their passage through the first-mentioned waveguide systems all the said additional waveguide systems are disposed so as to succeed the first-mentioned waveguide systems.

In order that the invention may readily be carried into effect, an embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawing.

The single figure of the drawing shows a perspective view of a high-frequency furnace according to the invention, which is designed for drying a tow of filaments l, which enters the high-frequency furnace with a degree of humidity, measured in percent by weight, of about 300%, and this degree of humidity is to be reduced to, for example, exactly 12% by high-frequency heating in thehigh-frequency furnace.

For the high-frequency heating the high-frequency furnace comprises three magnetron generators 2, 3, 4 of 2 kw. each for generating oscillations at a frequency of, for example, 2450 mc./s. The ultra-high frequency oscillations of each of the magnetron-generators 2, 3, 4 are applied through wave-guides 5, 6, 7 to individual ones of three folded waveguide systems 8, 9, 10 arranged in sequence.

To illustrate the structure of the identical waveguide systems 8, 9, 10, the outer wall of the waveguide system 10 is partially broken away to show that the folded waveguide system 10, which is contained in a box, is in the form of a plurality of adjacent lengths of waveguide which are joined to one another, each pair of adjoining lengths being separated from one another by a common boundary wall. Ultra-high frequency oscillations of the TE mode are fed into the folded waveguide systems 8, 9, 10 through the waveguides 5, 6, 7 and propagate in the folded waveguide systems 8, 9, 10 in a manner such that the direction of polarisation of the electric field vector is at right angles to the common boundary walls.

In order to enable the tow of filaments 1 required to be dried to be passed through, a channel 11 is provided in each of the folded waveguide systems 8, 9, 10 in a direction at right angles to the boundary walls of the adjoining lengths of waveguide, each wall being provided with a slit-shaped aperture of passage. The tow of filaments to be dried passes successively through the waveguide systems 8, 9, 10 fed by the magnetron generators 4, 3, 2. Short-circuit plungers 12, 13, 14 are arranged in the wave-guides 5, 6, 7 to provide load-matching for the magnetron generators 2, 3, 4.

When the tow of filaments 1 to be dried is thus led through the folded waveguide systems 8, 9, 10 in sequence, in each of said folded waveguide systems 8, 9, 10 the ultrahigh frequency oscillations of the TE mode are guided along the tow 1 in the successive lengths of waveguide so that the tow 1 is dried by the dielectric heating produced. Per unit of time the tow 1 when passing through the folded waveguide systems 8, 9, 10 absorbs a power which is proportional to the squareof the electric field strength and to the value of the instantaneous load.

It has been found that in the said drying process in the high-frequency furnace so far described the efficiency of heating was very poor. This was due to the fact that because of the reduction of the percentage of humidity in the tow 1 in the course of the drying process the load and hence the absorbed power have greatly decreased. For example, when the percentage of humidity has been reduced to 15% the load is only A of its initial value. To achieve a satisfactory efiiciency of heating, the folded waveguide systems 8, 9, 10 had to be made excessively long.

According to the invention the length of the highfrequency furnace is reduced and at the same time the heating efiiciency is improved by applying the ultra-high frequency'oscillations produced by the various magnetron generators 2, 3, 4 to the associated wave guide systems 10, 9, 8 through additional folded wave guide systems 15, 16, 17. Each of the latter wave guide systems also comprise a plurality of adjacent lengths of waveguide provided with apertures in their walls which form a channel 11 for guiding the object to be heated in a direction at right angles to the walls of the waveguides. In order to achieve additional high-frequency heating of the objects which have passed through the first-mentioned waveguide systems 8, 9, 10, the said additional waveguide systems 15, 16, 17 are arranged so as to succeed the firstmentioned waveguide systems 8, 9, 10. By way of illustration, the outer wall of the additional waveguide system 15 is partly broken away so as to show that this system, similarly to the waveguide system 10, comprises a plurality of lengths of waveguide joined to one another,

adjoining lengths being separated by a common boundary wall.

In the high-frequency furnace shown, the degree of humidity of the tow of filaments 1 is considerably reduced by the high-frequency heating in the folded waveguide systems 8, 9, 10, after which this tow 1 is passed through the additional waveguide systems 15, 16, 17 for further reduction of the degree of humidity to the desired value. For example, in the embodiment described the degree of humidity of the tow 1 after passing through the folded waveguide systems 8, 9, may be and is to be reduced to the desired value of 12% in the additional wave-guide systems 15, 16. 17.

In comparison with the folded waveguide systems 8, 9, 10 the load imposed on the waveguide systems 15, 16, 17 is considerably decreased owing to the reduction in the degree of humidity of the tow 1. However, in the wave guide systems 15, 16, 17 high-frequency heating is effected at the maximum field strength of the high-frequency oscillations due to their direct connection to the magnetron generators 2, 3, 4, and this maximum field strength is substantially maintained in the folded waveguide systems 15, 16, 17 owing to the slight dissipative power of the small load imposed by the tow 1 having a low degree of humidity. In the high-frequency furnace described, a particularly effective drying process is effected in the folded wave-guide systems 15, 16, 17 in spite of the small load imposed by the tow 1 having a low degree of humidity since, as set forth hereinbefore the absorbed power is proportional to the square of the field strength which has a maximum value in the folded Wave-guide systems 15, 16, 17. In addition, the efficiency of the magnetron generators 2, 3, 4 is particularly high because of the fact that the high-frequency oscillations after passing through the folded waveguide systems 15, 16, 17 are applied, through the waveguides 5, 6, 7, to the first-mentioned folded waveguide systems 8, 9, 11) for the magnetron generators 2, 3, 4 are each connected to one of the folded wave guide systems 15, 16, 17 and to one of the folded Waveguide systems 8, 9, 10, which represent a light load and a heavy load respectively, so that the overall loading of each of the magnetrons is favourable. The matching plungers 12, 13, 14 permit an optimum power transfer from the magnetron generators 2, 3, 4 to the tow of filaments 1.

In order to obtain even loading of the magnetron generators 2, 3, 4, it is of advantage to connect the first (15) of the additional waveguide systems 15, 16, 17 to the last (10) of the first-mentioned waveguide systems 8, 9, 10, the second (16) of the waveguide systems 15, 16, 17 to the last but one (9) of the additional waveguide systems 8, 9, 10 and the third (17) of the waveguide systems 15, 16, 17 to the last but two (8) of the additional waveguide systems 8, 9, 10.

Thus, the length of the high-frequency furnace is reduced while its heating efficiency is increased, and furthermore an effective drying process is achieved. In addition to the said advantages, the high-frequency furnace described provides the important practical effect of remarkable uniformity of the drying process. If, for ex- :ample, the tow of filaments 1 before passing through the waveguide systems 15, 16, 17 shows differences in degree of humidity along its width, at the area of the highest degree of humidity of the said tow 1 due to the increase in the load at this area the power absorbed at this area and hence the drying effect will greatly increase, since the absorbed power is proportional to the value of the load and to the square of the electric field strength, which has a maximum value in the waveguide systems 15, 16, 17. The folded waveguide systems 15, 16, 17 provide a highly equalising effect with respect to differences in degree of humidity and it has been found, for example, that the difference in degree of humidity of the tow 1 after passing through the high-frequency furnace are less than a few tenths of a percent.

The following data are given of a high-frequency furnace of the type described which has been extensively tested in practice:

Magnetron generators 2, 3, 4 kw 2 Length of the waveguide systems 8, 9, 10 cms 36 Length of the waveguide systems 15, 16, 17 cms 40 Width of the channel 11 cms 60 Height of the channel 11 mms 4 In the high-frequency furnace described, a tow of filaments 1 having a length of 3 kms. and a width of 50 cms. was treated per hour to reduce the degree of humidity from 300% to 12%. The efii-ciency of the furnace was The structure of the high-frequency furnace, in which high-frequency heating is performed at a maximum field strength with a low degree of humidity of the tow of filaments in the folded waveguide systems 15, 16, 17 and in which the ultra-high frequency oscillations taken from the waveguide systems 15, 16, 17 are used for high-frequency heating of the tow with a comparatively high degree of humidity in the folded waveguide systems 8, 9, 10, permits of further reducing the size of the high-frequency furnace, which is effected by reducing the spacings between the boundary walls of the lengths of waveguide in the waveguide systems 8, 9, 10.

This provides an increased field strength in the waveguide systems 8, 9, 1t), and it has been found that owing to the comparatively high load imposed on the waveguide systems 8, 9, 10 the spacings between the boundary walls of the lengths of waveguide in the said systems 8, 9, 19 may be reduced without reducing the efiiciency of heating. These spacings, which are 2 cms. in the waveguide systems 15, 16, 17, may be reduced to 1 cm. in the waveguide systems 8, 9, 10 without adversely affecting the heating efficiency.

What is claimed is:

1. High-frequency heating apparatus comprising first and second groups of heating units sequentially arranged in the path of an object to be heated, each of said heating units comprising a waveguide structure having a plurality of spaced partition members extending transversely to said path and arranged to form a plurality of adjacent waveguide sections interconnected to define a serpentine path for high-frequency energy, each of said partition members having an aperture therein to form a channel for the passage of said object, a plurality of high-frequency generators coupled to individual ones of the heating units of said first group, and means for coupling the heating units of said first group to individual ones of the heating units of said second group thereby forming a high-frequency energy propagation path for each generator com prising a heating unit of each of said first and second groups.

2. Apparatus as described in claim 1 wherein the heating units of said second group are arranged ahead of the heating units of said first group in the path of said object.

3. Apparatus as described in claim 2 wherein the first heating unit of said first group is coupled to the last heating unit of said second group, the second heating unit of said first group is coupled to the next to the last heating unit of said second group, and so on.

4. Apparatus as described in claim 2 wherein the partition members of said heating unit comprise parallel walls having aligned apertures therein arranged to allow said object to pass through said waveguide structures in sequence in a direction at right angles to the walls of the adjacent waveguide sections.

5. Apparatus as described in claim 2 wherein said heating units are arranged so that the space between adjacent partition members of the heating units of said second group are smaller than the space between adjacent partition members of the heating units of said first group.

6. Apparatus as claimed in claim 3 wherein said heating units are arranged so that the space between adjacent partition members of the heating units of said second group are smaller than the space between adjacent partition members of the heating units of said first group.

7. High-frequency heating apparatus comprising first and second equal groups of heating units sequentially arranged in the path of an object to be heated, all of the heating units of said second group being arranged ahead of the heating units of said first group so that said object passes through said second group of heating units before it passes through said first group of heating units, each of said heating units comprising a waveguide structure having a plurality of parallel spaced partition members extending transversely to said path and arranged to form a plurality of adjacent waveguide sections interconnected to define a serpentine path for high-frequency energy, said partition members having aligned apertures therein for passage of said object, a plurality of high-frequency generators equal to the number of heating units of said first group, means for coupling each of said generators to an individual one of the heating units of said first group, and energy coupling means interconnecting the References Cited by the Examiner UNITED STATES PATENTS 2,521,993 9/1950 Parker 219-1055 2,640,142 5/1953 Kinn 219-10.55 3,027,442 3/ 1962 Verstraten 219-l0.55

FOREIGN PATENTS 629,047 2/1962 Belgium. 1,370,675 7/ 1964 France.

982,171 2/ 1965 Great Britain.

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner. 

1. HIGH-FREQUENCY HEATING APPARATUS COMPRISING FIRST AND SECOND GROUPS OF HEATING UNITS SEQUENTIALLY ARRANGED IN THE PATH OF AN OBJECT TO BE HEATED, EACH OF SAID HEATING UNITS COMPRISING A WAVEGUIDE STRUCTURE HAVING A PLURALITY OF SPACED PARTITION MEMBERS EXTENDING TRANSVERSELY TO SAID PATH AND ARRANGED TO FORM A PLURALITY OF ADJACENT WAVEGUIDE SECTIONS INTERCONNECTED TO DEFINE A SERPENTINE PATH FOR HIGH-FREQUENCY ENERGY, EACH OF SAID PARTITION MEMBERS HAVING AN APERTURE THEREIN TO FORM A CHANNEL FOR THE PASSAGE OF SAID OBJECT, A PLURALITY OF HIGH-FREQUENCY GENERATORS COUPLED TO INDIVIDUAL ONES OF THE HEATING UNITS OF SAID FIRST GROUP, AND MEANS FOR COUPLING THE HEATING UNITS OF SAID FIRST GROUP TO INDIVIDUAL ONES OF THE HEATING UNITS OF SAID SECOND GROUP THEREBY FORMING A HIGH-FREQUENCY ENERGY PROPAGATION PATH FOR EACH GENERATOR COMPRISING A HEATING UNIT OF EACH OF SAID FIRST AND SECOND GROUPS. 